1. Field of the Invention
This invention pertains to drilling through the earth. More particularly, method and apparatus are provided for drilling through casings and then drilling extended drainholes from wells.
2. Description of Related Art
Oil and gas wells are normally drilled vertically from the surface of the earth to the depth of an oil or gas reservoir using a rotary drill. Metal casing is then placed in the wells and cemented in place. The metal casing is usually from about 41/2 inches to 8 inches in diameter. Although most wells are vertical, in recent years wells drilled in a horizontal direction have become common.
It is known to drill drainholes from a larger wellbore for the purpose of increasing production rate of oil or gas from a well. (Karlsson and Bitto, World Oil, p. 51 ff, March 1989) Drainholes have been used as alternatives to other techniques, such as hydraulic fracturing, for enhancing production rate of oil or gas from wells. The direction of hydraulic fractures is controlled by stresses in the earth, and the direction in a particular reservoir may not be optimal for recovering hydrocarbons. Drainholes, however, can be drilled in a selected direction. This may be particularly advantageous when natural fractures are present in a hydrocarbon reservoir or for other reasons. (Kulch, World Oil, p. 47 ff, September 1990).
Significant developments have occurred in recent decades concerning the understanding, improved efficiency, and better methods for using jet drills for cutting rock. U.S. Pat. No. 4,119,160 discloses an apparatus and method that uses two nozzle jets, one axially straight ahead and the other at an angle of 30 degrees with respect to straight ahead. The nozzle is mechanically rotated. Compared to a single jet, this rotating two-jet arrangement cuts much more rapidly and efficiently. In their attempt to prevent contact of the central cone of the drill by adding a small 0.02 inch central orifice, they observed a huge 750% increase of rock removal with only a 25% increase in fluid flow rate. They performed experiments on harder rock specimens and concluded that the uniaxial compressive strength of the rock is not an adequate measure of its cuttability by water jets. U.S. Pat. 4,346,761 discloses an apparatus using high-pressure (about 3,000 psi) abrasive fluid jets to perforate steel casings. Extension of the jets into the reservoir is limited. U.S. Pat. No. 5,291,956 discloses a low-cost method of drilling and completing wells using a nonrotating jet drilling tool and coiled tubing. It uses a pipe assembly including a rigid bent pipe capable of plastically deforming the tube when the tube is forced from a first end to a second end of the bent pipe. A residual bend-remover straightens the stainless steel coiled tubing as it passes through the tube and into the formation. U.S. Pat. Nos. 5,413,184 and 5,853,056 disclose method and apparatus for penetrating a well casing and the surrounding earth strata. Rotary drilling using a ball drill and a hydraulic motor penetrates the casing. This equipment is then withdrawn from the well and a flexible tube with a nozzle on its end is inserted into the well to drill a horizontal extension into the reservoir. It is necessary to reinsert the jet drill in the hole originally made by the ball cutter.
What is needed is apparatus and method for perforating the casing and continuing to drill rapidly an extended lateral borehole or drainhole into the reservoir. Further, control of the direction of a jet drill as it drills through the reservoir and a method for measuring or detecting the location of the drill are significant improvements.